1. Field of the Invention
The present invention relates generally to telecommunications, and more specifically, to wireless communications.
2. Background
In a reverse link scheme of a system based on the CDMA 2000 1xEV-DO standard according to “3rd Generation Partnership Project 2 ‘3GPP2’ CDMA2000 High Rate Packet Data Air Interface Specification,” 3GPP2 C.S0024 Version 3.0, Dec. 5, 2001, a plurality of access terminals (ATs) are allowed to access a base transceiver station (BTS) in an access network (AN) simultaneously. The reverse link data rate of each access terminal in the state of the art ranges from 9.6 kilobits per second (kbps) to 153.6 kbps and is controlled by using a closed-loop rate adaptation (CLRA) algorithm which is known to a person skilled in the art.
The data rates for the access terminals communicating with the base transceiver station vary depending on the status of a reverse activity (RA) bit which is set by the base transceiver station and transmitted to the access terminals. According to the CDMA 2000 1xEV-DO standard, if the RA bit is set to 1, all access terminals communicating with the base transceiver station are instructed to lower their data rates to maintain system stability. If the RA bit is set to 0, then the access terminals may increase their data rates as long as sufficient transmit power is available to support the increased data rates.
The base transceiver station typically sets the RA bit based on the maximum ratio I0/N0 per antenna, where I0 is the total power received by an antenna and N0 is the thermal noise power. In general, the I0/N0 ratio, which is also called the rise-over-thermal (ROT) ratio, serves as an indication of the level of user-to-user interference in a wireless communication system. A higher ROT ratio is usually indicative of a higher level of user-to-user interference.
If the ROT is higher, access terminals with good link conditions typically operate at higher data rates, and consequently all access terminals operate at higher transmit power. Excessive power transmitted by all access terminals may cause excessive multiple-access interference (MAI) at the base transceiver station, thereby destabilizing the power control loop and consequently resulting in outage situations for access terminals near the sector edge of the base station antenna.
A typical scheme of limiting the transmitted power of the access terminals to prevent excessive MAI is to limit the ROT such that the ROT is below a certain threshold. In a conventional CDMA 2000 1xEV reverse link, the limit on the ROT is accomplished by setting the RA bit and transmitting the RA bit to the access terminals. For example, if the ROT exceeds a certain threshold, the base transceiver station sets the RA bit to 1 to signal the access terminals to lower their data rates. Otherwise, the RA bit is set to 0. The RA bit is typically set at predetermined intervals, for example, once every sixteen slots.
In the current CDMA 2000 1xEV system, the ROT threshold is a fixed number that is predetermined before the base transceiver station is operational. The ROT threshold is typically calibrated for a certain operating environment for the base transceiver station and set to a fixed value which theoretically would maximize the system throughput while minimizing the probability of outages for access terminals near the sector edge. In general, an ROT threshold that is set at an excessively high value may lead to system instability and cause excessive outages and poor performance for users near the cell edge, whereas an ROT threshold that is set too conservatively forces access terminals to reduce their data rates unnecessarily, thereby suppressing the overall system throughput.
In many practical situations, the radio channels of a typical base transceiver station are dynamic. The interference conditions for a typical base transceiver station depend not only on a fixed sector layout but are also affected by various factors such as time-varying shadowing, multipath fading, and the positions and movements of access terminals communicating with the base transceiver station. Therefore, it is nearly impossible to set the ROT threshold at a fixed optimal value which ideally would maximize the system throughput while minimizing the multiple-access interference at all times. In many practical situations, the ROT threshold is typically set conservatively to limit outages in the worse-case scenario, thereby leading to lower than desired system throughput when no significant multiple-access interference is present.
Therefore, there is a need in the art for a scheme for dynamically setting the ROT threshold during the operation of the base transceiver station in response to time-varying interference conditions.